This invention relates to information storage and retrieval apparatus and, more particularly, to associative memory apparatus.
An associative memory, in which pairs of information patterns are stored in such a way that introduction of one pattern results in the recall of another, has potential applications in logical operations, pattern recognition, and image understanding.
Various types of associative memories have been implemented and reported in the literature. One approach to implementing such memories employs various algorithms for associative recall using matrix algebraic manipulations on a digital computer. Another approach employs holograms, which are inherently optical associative memories in that input reference beams are used to recall associated images.
Recently, a new matrix algebra based associative memory model was described by J. J. Hopfield in his paper "Neural Networks and Physical Systems with Emergent Collective Computational Abilities," Proc. Natl. Academy of Science U.S.A., 1982, Vol. 79, pp.2554-2558. The Hopfield model utilizes feedback and nonlinear thresholding to force the output pattern to be the stored pattern which most closely matches the input pattern. A major drawback of this model is the large storage and computational effort required for manipulation of an association matrix used in the model. In the instance of storing two dimensional image patterns, a four dimensional matrix is required.
An implementation of the Hopfield model employing optical vector-matrix multiplication has been reported by D. Psaltis and N. Farhat in their paper "A New Approach to Optical Information Processing Based on the Hopfield Model," Optical Society of America Annual Meeting, San Diego, Oct. 1984. Psaltis and Farhat successfully demonstrated the associative recall of one dimensional binary vectors. They also suggested an alternative holographic implementation requiring the use of two identical holograms and a bistable optical amplifier. The input image illuminates the first hologram, generating a reference wave. The reference wave in turn, illuminates the second hologram, generating an estimate of the input image. This estimate is applied to the bistable optical amplifier where portions of it exceeding a predetermined threshold are amplified and directed to illuminate the first hologram along with the input image.
Their specific implementation is limited to the processing of black and white (binary) image information as opposed to the processing of gray-scale image information, and also requires a large area bistable optical amplifier as well as two holograms.
Accordingly, it is an object of the present invention to provide a new associative holographic memory employing only a single hologram.
It is another object of the invention to provide a new associative holographic memory capable of processing gray scale and/or phase encoded image information.
It is yet another object of the invention to provide a new associative holographic memory employing phase conjugate mirrors and a two-wave mixing contra-directional coherent image amplifier to provide rapid convergence to the desired image.